User manual | CHAPTER 5 PERENNIAL DRYLAND PASTURE - WEEPING LOVEGRASS Eragrostis curvula

CHAPTER 5
PERENNIAL DRYLAND PASTURE - WEEPING LOVEGRASS
(Eragrostis curvula L.)
The following hypotheses are tested in this chapter.
HYPOTHESIS 1
•
Sludge loading above the 8 Mg ha-1 yr-1 norm could improve weeping
lovegrass hay yield, crude protein content, and water use efficiency.
HYPOTHESIS 2
•
The ideal sludge loading rate to satisfy weeping lovegrass N demand is
dynamic and could exceed the 8 Mg ha-1 yr-1 sludge norm.
HYPOTHESIS 3
a. Under high hay yield production conditions, N supply from double the norm
can be fully utilized and are not prone to excessive nitrate leaching.
HYPOTHESIS 4
•
Sludge application according to weeping lovegrass N demand results in
the accumulation of total and Bray-1P in the soil profile.
178
5.1 Hay yield, crude protein content, and water use efficiency
5.1.1 Hay yield
Doubling of the sludge upper limit norm significantly improved weeping lovegrass
annual hay yield except during 2005/06, where the difference was not statistically
significant (Table 5.1). Hay yield was generally higher for the first cut compared
with the second each year, except for the 2007/08 growing season, where the
second cut was higher than the first (Table 5.2). It was interesting to note that
weeping lovegrass annual hay yield from the 8 Mg ha-1 yr-1 sludge treatment was
similar to that of the inorganic fertilizer. The only exception was during the wet
2007/08 growing season, where hay yield from the inorganic fertilizer was
significantly higher than the 8 Mg ha-1 treatment.
Table 5.1 Annual hay yield of weeping lovegrass as affected by three sludge
application rates, inorganic fertilizer, and control.
Weeping lovegrass hay yield
Treatments
2004/05
2005/06
2006/07
2007/08
Mg ha-1
Control†
10.18
9.08
6.97
8.15
10.74
9.64
10.05
12.70
14.10
12.50
11.87
14.90
16 Mg ha yr
15.08
12.78
12.36
17.31
Inorganic fertilizer ‡
13.78
12.56
11.78
16.50
LSD (5%)
0.61
0.43
0.46
0.73
CV, %
3
3
3
3
4 Mg ha-1 yr-1
-1
-1
8 Mg ha yr
-1
-1
† Zero sludge and inorganic fertilizer applied
‡ 200 kg N and 80 kg P per hectare per annum applied
179
In general, hay yield throughout the study period was much higher than the long
term average values of 6 Mg ha-1 reported by Dickinson et al. (2004) from Hutton
soils receiving similar annual rainfall. This is most probably due to a better
nutrient status of the soil from this study compared with the soils reported by
Dickinson et al. (2004).
The relatively low hay yield from the second cut compared with the first cut of
each year was mainly attributed to the relatively low rainfall experienced during
the second half of the season (Fig. 5.1). This is because a relatively higher
rainfall experienced during the second cut of the 2007/08 growing season
rendered higher forage yield than the first cut. The amount of rainfall and
weeping lovegrass hay yield of a control and three sludge rate treatments are
presented in a 3 D graph (Fig. 5.2). The graph vividly shows that weeping
lovegrass hay yield response to sludge rate increases with increase in rainfall.
This coincides with the explanation by Miles and Manson (2000) and Dickinson et
al. (2004), who report that weeping lovegrass response to N increases with
increase in the availability of water (rainfall).
180
Table 5.2 Weeping lovegrass hay yield per cut of three sludge application rates, an inorganic fertilizer, and a
control during the 2004/05 to 2007/08 growing seasons.
† Zero sludge and inorganic fertilizer applied
Weeping lovegrass hay yield per cut
Treatments
2004/05
First
Second
cut
cut
2005/06
First
Second
cut
cut
2006/07
First
Second
cut
cut
2007/08
First
Second
cut
cut
Mg ha-1
Control†
6.69
3.49
4.96
4.12
4.09
2.88
4.00
4.15
4 Mg ha-1 yr-1
6.97
3.77
5.51
4.13
5.96
4.09
5.94
6.76
8 Mg ha-1 yr-1
8.65
5.45
7.39
5.11
7.34
4.53
6.12
8.78
16 Mg ha-1 yr-1
9.14
5.94
7.66
5.21
7.74
4.62
6.67
10.64
Inorganic fertilizer‡
8.49
5.29
7.44
5.12
7.16
4.62
7.16
9.34
LSD (5%)
0.41
0.25
0.26
0.26
0.26
0.31
0.38
0.64
4
3
5
4
5
CV, %
3
3
3
‡ 100 kg N and 40 kg P per hectare per annum per cut applied
181
450
400
Rainfall (mm)
350
300
250
200
150
100
50
0
First cut Second First cut Second First cut Second First cut Second
cut
cut
cut
cut
2004/05
2005/06
2006/07
2007/08
Growing year
Figure 5.1 Rainfall distribution during the first and second cuts of weeping
lovegrass planted during the 2004/05 to 2007/08 growing seasons, at ERWAT,
Ekurhuleni district, South Africa.
182
20
Hay yield (Mg ha-1)
16
16-20
12
12-16
8
8-12
4-8
4
16
0
8
385
562.6
Rainfall (mm)
4
564.8
706.5
0
Sludge rate
(Mg ha-1 yr-1)
Figure 5.2 Weeping lovegrass hay yield as affected by rainfall amount and
sludge application rate.
5.1.2 Crude protein content
Doubling of the annual upper sludge limit improved weeping lovegrass crude
protein content, except for the second cuts of the 2006/07 and 2007/08 growing
seasons (Table 5.3). It was also evident that the level of crude protein from the
second cut of each year was lower than that of the first. Crude protein content is
the most limiting constituent for animal performance feeding on pasture and was
estimated by multiplying hay N content by a factor of 6.25 as reported by
Meissner et al. (2000).
183
The increase in the level of crude protein with doubling of the 8 Mg ha-1 sludge
limit was mainly attributed to the increase in the availability of N, because crude
protein of weeping lovegrass increases with fertilization under nutrient limiting
conditions (Masters and Britton, 1990). The low crude protein observed during
the second cut of the first three years was mainly due to the low rainfall, because
weeping lovegrass crude protein content decreases under drought conditions
(McFarland, 1999). This is most probably due to the translocation of N from
leaves to roots, which is common in warm season grasses during drought
periods (Heckathron and Delucia, 1999). The low crude protein content observed
during the second cut of 2008 was, however, mainly attributed to a dilution effect
as a result of the high biomass production (Table 5.2, column 9).
Generally, the crude protein levels of all the treatments were within the ranges
reported by Strickland (1973) for weeping lovegrass (63-175 g kg-1). It was also
above the minimum crude protein requirements of ruminants (70-80 g kg-1)
Meissner et al., (2000), except for the second cut of the control treatment during
the 2007/08 growing season.
184
Table 5.3 Crude protein content of weeping lovegrass as affected by three sludge application rates, an inorganic
fertilizer treatment, and a control.
† Zero sludge and inorganic fertilizer applied
Weeping lovegrass crude protein content
Treatments
2004/05
First
Second
cut
cut
2005/06
First
Second
cut
cut
2006/07
First
Second
cut
cut
2007/08
First
Second
cut
cut
g kg-1
Control†
121
94
96
79
106
85
77
62
4 Mg ha-1 yr-1
126
98
100
86
107
89
104
77
8 Mg ha-1 yr-1
128
102
100
89
108
85
104
77
16 Mg ha-1 yr-1
155
112
102
99
112
85
107
77
Inorganic fertilizer‡
136
106
104
94
109
85
107
82
4
3
3
4
6
1
3
1
2
3
2
2
2
1
LSD (5%)
CV, %
2
2
‡ 200 kg N and 80 kg P per hectare per annum applied
185
5.1.3 Effect of sludge application rate on rainfall use efficiency
Sludge applied at double of the upper limit norm significantly improved the annual
rainfall use efficiency compared with lower rates (Table 5.4). The significant
improvement in annual water use efficiency was due to the significant increase in
hay yield per unit water used for each cut, or for one of the two cuts in a season
(Table 5.5). Rainfall use efficiency (RUE) is a factor which indicates the
productivity of an ecosystem (Guevara et al., 2005). This depends on soil and
vegetation condition and its dynamic status (Le Houérou, 1984).
During the 2006/07 growing season, the water use efficiency of treatments that
received 8 and 16 Mg ha-1 sludge and an inorganic fertilizer was highest
compared with similar treatments during the other three growing seasons, which
experienced higher rainfall and hay yield. Nevertheless, each treatment had
relatively lower hay yield compared with similar treatment due to the low rainfall
experienced during the specified year. Generally, the rainfall use efficiency from
this study was much higher than the ranges reported by Guevara et al. (2005) for
weeping lovegrass (3.7-10 kg DM ha-1 year-1 mm-1) in Argentina. This is most
probably due to higher water holding capacity and nutrient status of the soil from
this study among other factors.
186
Table 5.4 Annual rainfall use efficiency of weeping lovegrass as affected by three
sludge application rates, an inorganic fertilizer, and a control.
Annual rainfall use efficiency
Treatments
2004/05
2005/06
2006/07
-1
2007/08
-1
kg ha mm
Control†
18
16
18
12
4 Mg ha-1 yr-1
19
17
26
18
8 Mg ha-1 yr-1
25
21
31
21
16 Mg ha yr
27
23
33
24
Inorganic fertilizer‡
24
22
31
23
LSD (5%)
1
1
1
1
CV, %
3
2
3
4
-1
-1
† Zero sludge and inorganic fertilizer applied
‡ 200 kg N and 80 kg P per hectare per annum applied
To summarize, sludge applied at double the norm improved weeping lovegrass
hay yield, crude protein content, and water use efficiency compared with lower
rates under the prevailing climatic conditions. Therefore, the following hypothesis
was accepted:
•
Sludge application above the 8 Mg ha-1 yr-1 norm could improve dryland
pasture (weeping lovegrass) hay yield, crude protein content, and water
use efficiency.
187
Table 5.5 Rainfall use efficiency of weeping lovegrass per cut as affected by three sludge application rates, an
inorganic fertilizer, and a control.
† Zero sludge and inorganic fertilizer applied
Weeping lovegrass seasonal rainfall use efficiency
Treatments
2004/05
First
Second
cut
cut
2005/06
First
Second
cut
cut
2006/07
First
Second
cut
cut
2007/08
First
Second
cut
cut
kg ha-1 mm-1
Control†
19
17
16
17
17
21
13
11
4 Mg ha-1 yr-1
19
18
17
17
25
29
19
17
8 Mg ha-1 yr-1
24
26
22
21
30
33
19
23
16 Mg ha-1 yr-1
26
28
24
22
32
33
21
27
Inorganic fertilizer‡
24
25
23
21
30
33
22
24
1
1
1
1
1
2
1
2
3
4
3
4
5
6
LSD (5%)
CV, %
3
4
‡ 200 kg N and 80 kg P per hectare per annum applied
188
5.2 Hay N uptake
Weeping lovegrass annual hay N uptake increased significantly with doubling of
the annual upper sludge application limit (Table 5.6). Annual hay N uptake from
the control treatment tended to decrease over time. Other treatments, however,
did not show a specific trend across years. Crop N uptake was higher during the
first cut than the second, except in 2008, where the second cut was higher for the
8 and 16 Mg ha-1 yr-1 sludge treatments (Table 5.7).
Table 5.6 Annual weeping lovegrass N uptake from three sludge application
rates, inorganic fertilizer treatment, and a control during the 2004/05 to
2007/08 growing seasons.
Weeping lovegrass hay N uptake
Treatments
2004/05
2005/06
2006/07
2007/08
kg ha-1
Control†
182
129
109
91
-1
200
145
160
182
8 Mg ha-1 yr-1
266
192
188
210
16 Mg ha yr
333
208
202
245
Inorganic fertilizer‡
275
201
189
245
LSD (5%)
10
8
8
10
3
3
3
3
-1
4 Mg ha yr
-1
CV, %
-1
† Zero sludge and inorganic fertilizer applied
‡ 200 kg N and 80 kg P per hectare per annum applied
189
1
2
Table 5.7 Weeping lovegrass hay N uptake per cut from three sludge application rates, inorganic fertilizer
treatment, and a control
Weeping lovegrass hay N uptake
2004/05
First
Second
season
season
Treatment
2005/06
First
Second
season
season
2006/07
First
Second
season
season
2007/08
First
Second
season
season
kg ha-1
Control†
130
52
76
53
70
40
50
41
-1
4 Mg ha yr
141
60
88
57
102
58
99
84
8 Mg ha-1 yr-1
177
89
118
73
127
62
102
108
16 Mg ha-1 yr-1
227
106
125
83
139
63
115
131
Inorganic fertilizer‡
185
90
124
77
126
63
123
123
LSD (5%)
8
3
5
5
6
4
7
8
CV, %
3
3
3
5
4
5
5
6
-1
3
4
5
† Zero sludge and inorganic fertilizer applied
‡ 200 kg N and 80 kg P per hectare per annum applied
6
7
8
190
The reported increase in N uptake for every increment in sludge rate depicts the
increase in the availability of nitrogen. This is because nitrogen uptake is a more
sensitive indicator of nitrogen availability from nitrogenous fertilizer sources
(Kiemnec et al., 1987). This increase in plant N availability enhanced weeping
lovegrass hay yield, because N is considered the key element for dry matter
production (Miles and Manson, 2000) and dry matter production of weeping
lovegrass increases with the availability of nitrogen (Rethman et al., 1984). In
contrast, the decline in N uptake over time for the control treatment was mainly
due to the decline in soil N availability.
Assuming that 50 percent of the organic N is released during the first year of
sludge application (Snyman and Herselman, 2006) followed by 8, 3, 1, and 1
percent of the organic N applied during the first year being released during the
second, third, fourth, and fifth years, respectively (Sullivan and Cogger, 2000).
The four year mean N uptake by the 16 Mg ha-1 sludge treatment was equivalent
to an N supply from a 15.8 Mg sludge ha-1 during the first year of application (Fig.
5.3). The rate decreased during the following 8 years and reached an equilibrium
application rate of 13.2 Mg ha-1 from the 10th year on. If the sludge N content was
3.85%, the annual sludge application rate could have equilibrated at 8.8 Mg ha-1
from the 10th year on (Fig. 5.3). The decline in sludge application rate across
years was mainly due to the N carry over effects from previous year’s
applications.
191
18
16
Sludge N content (2.56 %)
Sludge N content (3.85%)
Sludge application rate (Mg ha-1)
14
12
10
8
6
4
2
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Years
Figure 5.3 Sludge application rate to satisfy four year mean weeping lovegrass N
demand (247 kg N ha-1) as affected by sludge N content and N carry over effects.
The higher hay N uptake during the first cut compared with the second was
mainly attributed to the availability of higher rainfall during the first cut of the year
(Fig. 5.1). The event during the 2007/08 growing season, where higher rainfall
during the first cut from the 8 and 16 Mg ha-1 sludge treatments produced higher
yield than the second proved this reality. The availability of water plays a
significant role in the mineralization of organic nitrogen (Jansson and Persson,
1982; Vinten and Smith, 1993), nitrification of ammonium (Breuer et al., 2002)
and crop uptake (Mackay and Barber, 1985).
192
Therefore, the following hypothesis was accepted:
•
The ideal sludge application rate to satisfy dryland pasture (Weeping
lovegrass) N demand is dynamic and could exceed the 8 Mg ha-1 yr-1
sludge norm.
5.3 Soil profile total N mass balance, nitrate leaching, residual
nitrate and ammonium
5.3.1 Total N mass balance
Based on the mass balance calculation of N imported with sludge less N exported
with hay, total N from sludge applied according to the 8 Mg ha-1 limit was close to
being but not quite sufficient to satisfy weeping lovegrass N demand (net negative
mass balance) (Table 5.8). Negative mass balances of all but the 16 Mg ha-1
treatment shows that the crop utilized N from the soil reserve. Therefore, total N
supply from sludge with 2.56% mean N applied according to the 8 Mg ha-1 norm
is not sufficient for optimal weeping lovegrass hay production under the prevailing
climatic and soil conditions. Doubling of the upper limit sludge norm, however,
resulted in a net positive mass balance. This indicates that the total N supply
surpassed hay N uptake. The excess N from double the norm was accumulated
mainly in the 0-0.1 m soil layer, because the sludge was surface applied (Fig.
5.4).
193
Table 5.8 Cumulative N supply (CUM NS)), uptake (CUM NU), and mass balance
of a weeping lovegrass treated with three sludge application rates, inorganic
fertilizer, and a control.
Four years cumulative mass balance
Treatment
CUM-NS
CUM-NU
CUM-NS
less
CUM-NU
Change
in soil N
storage
Mass
balance
difference
kg ha-1
Control†
0
511
-511
-600
89
-1
-1
409
687
-278
-381
103
-1
-1
818
856
-38
-203
165
1637
988
649
330
319
800
910
-110
-271
161
4 Mg ha yr
8 Mg ha yr
16 Mg ha-1 yr-1
Inorganic fertilizer‡
† Zero sludge and inorganic fertilizer applied
‡ 200 kg N per hectare per annum applied
Soil profile sampling to a depth of 0.8 m (change in soil storage) (Table 5.8)
showed similar trends to the supply less uptake mass balance. According to this
profile analyses, doubling of the annual sludge upper limit resulted in a net
positive mass balance while sludge applied according to the norm had a net
negative mass balance. Interestingly, however, there was a net positive
difference between the two mass balances: supply less uptake (CUM_NS less
CUM-NU) and change in storage between final and initial soil profile N contents
(Chance in soil N storage). This difference increased with increase in sludge
application rate (Mass balance difference) (Table 5.8).
The most probable cause for the mass balance difference was ammonia
volatilization losses, which is not accounted for in the N import less export mass
194
balance. This is because the sludge used in this study was anaerobically
digested with about 20-25% of total N in ammonium form. In addition, the sludge
was surface applied. Previous studies conducted by Adamsen and Sabey (1987)
showed that 40.3% of the NH4-N from surface applied sludge could be lost as
NH3 gas during the first two weeks of its application. This was in contrast to
0.35% loss from an incorporated sludge reported in the same study. Similarly,
other studies have also shown that on average 89% of the initial ammonium
could be lost in the form of ammonia gas from a surface applied anaerobically
digested sludge (Henry et al., 1999). Other possible sources of the differences
include sampling errors, N content variation within the sludge matrix, soil
heterogeneity, and probably denitrification (which is not dominant under dryland
cropping), and leaching (which could have been insignificant due to the low
rainfall experienced).
Comparison of mass balance differences was conducted between dryland maize
forage production, where sludge was incorporated immediately after application,
(Table 4.10) and weeping lovegrass, sludge surface applied, (Table 5.8). The
mass balance difference of weeping lovegrass for the 8 and 16 Mg ha-1 sludge
treatments was more than four times higher than similar treatments under
dryland maize conditions. In addition, the wetting front detectors buried at 0.3 m
depth responded only four times during the four year study period, each event
with nitrate concentrations of less than 14 mg L-1. This implies the presence of
high ammonia volatilization losses from surface application under the weeping
195
lovegrass production, supporting the previous argument on the most probable
reason for the mass balance difference.
Total N (%)
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0
0.1
Soil depth (m)
0.2
Initial
Control
4 t/ha/yr
8 t/ha/yr
16 t/ha/yr
Inorganic fertilizer
0.3
0.4
0.5
0.6
0.7
0.8
Figure 5.4 Initial soil profile total N and after four years of study with three sludge
rates (4, 8, and 16 Mg ha-1 yr-1), an inorganic fertilizer (200 kg N ha-1 yr-1), and a
control.
196
5.3.2 Residual nitrate and nitrate leaching
Residual nitrate in the top 0.5 m soil layer of all treatments including sludge
applied at double the norm remained less than the initial amount throughout the
study period (Table 5.9). The reduction in nitrate content of all treatments was
evident in the 0-0.3 m soil stratum, while the content in the 0.3-0.8 m layer
remained similar to initial values (Fig. 5.5). It was also evident that sludge applied
at double the norm had relatively lower residual nitrate than lower sludge rates in
2005/06, but higher at the end of 2007/08 growing season growing season.
Table 5.9 Residual nitrate mass in the top 0.5 m soil stratum of weeping
lovegrass plots treated with three sludge rates, an inorganic fertilizer, and a
control treatment.
Treatment
Control†
4 Mg ha-1 yr-1
8 Mg ha-1 yr-1
16 Mg ha-1 yr-1
Inorganic fertilizer‡
After 2nd hay cut
in 2005/06
kg ha-1
60
62
52
51
33
Initial
96
96
96
96
96
After 2nd hay cut
in 2007/08
18
18
16
58
51
† Zero sludge and inorganic fertilizer applied
‡ 200 kg N and 40 kg P per hectare per annum applied
Although residual nitrate at the end of the study was relatively higher for the 16 Mg
ha-1 yr-1 sludge treatment than other treatments, it was still low and was almost
half of the initial mass. The main reason for monitoring residual nitrate after
harvest was that nitrate leaching under dryland conditions usually takes place in
the beginning of the rainy season, especially before active root nutrient uptake in
197
the presence of high rainfall and residual soil nitrate. Nitrate is susceptible to
diffusion and transport through mass flow with soil water because there is little
tendency for soil colloids to absorb nitrate, which is negatively charged, as they
mostly posses a net negative charge (Cameron and Haynes, 1986).
Nitrate (mg kg-1)
0
5
10
15
20
25
30
35
0
Soil depth (m)
0.1
0.2
0.3
Initial
Control
4 t/ha/yr
8 t/ha/yr
16 t/ha/yr
Inorganic fertilizer
0.4
0.5
0.6
0.7
0.8
Figure 5.5 Residual nitrate before treatment application (initial) and after four
consecutive years of treatment application (three sludge rates, inorganic fertilizer
and control).
In this specific study the mean annual rainfall was low (405 mm in 2007) to
moderate (710 mm in 2008). The wetting front detectors buried at 0.3 m depth
responded only four times during the four year study period. The dates of
response and the corresponding nitrate concentrations from the 16 Mg ha-1 yr-1
198
sludge treatment were: 21/02/2005 (10 mg L-1 NO3), 01/03/2006 (14 mg L-1 NO3),
22/01/2008 (8 mg L-1 NO3), and 16/03/2008 (11 mg L-1 NO3). The control and 8
Mg ha-1 yr-1 sludge treatments had similar or lower nitrate concentrations during
the same time period. None of the WFDs buried at 0.6 m, however, responded.
Therefore, considering the low to moderate rainfall experienced during the study
period, low residual nitrate, and low nitrate concentrations collected during very
few events might indicate that nitrate leaching was minimal during the study
period.
5.3.3 Residual ammonium
Residual ammonium in the top 0.5 m soil stratum remained equal or less than
initial values for all treatments (Table 5.10). Residual ammonium increased with
increase in sludge application rate at the end of the 2007/08 growing season,
though it did not have any specific trend in 2005/06.
The low residual ammonium mass reported at the end of the study, despite a net
positive total N mass balance, might indicate that the ammonium added from the
sludge was either nitrified or taken up by the plants. The possibility for
ammonium to leach below the active root zone (0-0.6 m) in this specific soil type
under the prevailing rainfall and sludge rates is unlikely, considering the following
facts. Primarily the predominantly negatively charged soil clay and organic matter
particles can fix ammonium through the process of cation exchange. Secondly
199
ammonium can easily be immobilized by microbial biomass, and lastly,
ammonium is unstable and is readily nitrified (Cameron and Haynes, 1986).
Table 5.10 Residual ammonium mass in the top 0.5 m soil stratum after every
second weeping lovegrass hay cut during the 2004/05 to 2007/08 growing
seasons.
Treatment
Initial
After 2nd hay cut
After 2nd hay cut in
in 2005/06
2007/08
kg ha-1
Control†
65
16
20
4 Mg ha-1 yr-1
65
41
38
8 Mg ha-1 yr-1
65
31
46
16 Mg ha-1 yr-1
65
36
65
Inorganic fertilizer‡
65
59
47
† Zero sludge and inorganic fertilizer applied
‡ 200 kg N and 40 kg P per hectare per annum applied
Despite the net positive total N mass balance of the 16 Mg ha-1 yr-1 sludge
treatment (Table 5.8), residual ammonium and nitrate remained similar or less
than initial values. This indicates that a large fraction of the N in the soil is present
in organic form.
In conclusion, sludge applications that produced the highest weeping lovegrass
hay yield are not necessarily in total N balance. In this study, the 16 Mg ha-1 yr-1
sludge treatment significantly improved weeping lovegrass hay yield but was not
in total N mass balance. The same treatment accumulated an extra 649 kg N ha-1
200
(supply less uptake) or 330 kg N ha-1 (final less initial soil N storage change)
following four consecutive years of sludge application and hay harvest events.
Despite this net positive total N mass balance, there was neither residual nitrate
nor ammonium accumulation in the soil profile above the initial values. In addition,
the few soil solution samples collected from 0.3 m deep WFDs presented low
nitrate concentrations. The implications are that sludge applications of up to 16
Mg ha-1 yr-1 should have minimal environmental impacts through nitrate leaching
in the medium-term (four years) for weeping lovegrass hay production under the
prevailing climatic and edaphic conditions of the study site.
Therefore, the following hypothesis was accepted:
•
Sludge applications of up to 16 Mg ha-1 yr-1 that produced the highest hay
yield under the prevailing climatic and edaphic conditions were not prone
to excessive nitrate leaching in the medium-term (four years).
The following hypothesis was rejected:
•
Under high hay yield production conditions, N supply from double the norm
can be fully utilized.
Nevertheless, long term model simulations should be conducted because the
mineralization of N from sludge could take several years before reaching steadystate conditions.
201
5.4 Total P mass balance and residual Bray-1P
5.4.1 Total P mass balance
Sludge applications of all rates resulted in a net positive total P mass balance
(CUM-PS less CUM-PU) following four years of sludge applications and weeping
lovegrass hay harvest events (Table 5.11, columns 4 and 5). Based on the mass
balance calculation of total P imported with sludge less exported with weeping
lovegrass hay yield, sludge applications of 4 Mg ha-1 yr-1 supplied a cumulative
excess of 279 kg P ha-1 during the four year study.
Table 5.11 Cumulative total P supply (CUM-PS), uptake (CUM-PU), and mass
balance of a weeping lovegrass treated with three sludge rates, inorganic
fertilizer, and a control
Four years cumulative total P mass balance
Treatment
CUM-PS
CUM-PU
CUM-PS
less
CUM-PU
Mass
Change
in soil P balance
storage difference
kg ha-1
Control†
4 Mg ha-1 yr-1
-1
-1
8 Mg ha yr
-1
-1
16 Mg ha yr
Inorganic fertilizer‡
0
59
-59
-50
-9
352
73
279
302
-23
705
93
612
660
-48
1409
102
1307
1316
-9
160
93
67
96
-29
† Zero sludge and inorganic fertilizer
‡ 40 kg P ha-1 per year applied
Soil sampling to a depth of 0.8 m (Change in soil P storage) also supported after
trends for the total P supply less uptake mass balance (Table 5.11). Most of the
202
excess P added with the sludge was located in the 0-0.1 m soil layer with a slight
increase in the deeper 0.1-0.3 m soil layer (Fig. 5.6). The slight total P increment
in this soil layer was most probably due to the physical migration of colloidal
sludge particles between cracks formed during dry periods of the year, or from
preferential flow of particulate P (Jensen et al., 2000; Brock et al., 2007). The
grave concern with P surface accumulation is the potential threat to surface water
bodies through transport by runoff, enhancing the rate of eutrophication in fresh
water bodies (Carpenter et al., 1998).
Total P (%)
0
0.02 0.04 0.06 0.08
0.1
0.12 0.14 0.16 0.18
0
0.1
Soil depth (m)
0.2
Initial
Control
4 t/ha/yr
8 t/ha/yr
16 t/ha/yr
Inorganic fertilizer
0.3
0.4
0.5
0.6
0.7
0.8
Figure 5.6 Initial soil profile total P and after four consecutive years of treatment
applications in a weeping lovegrass hay production trial.
203
There was a mass balance difference between the supply less uptake mass
balance (Table 5. 11, column 4) and the final less initial soil profile total P mass
balance (Table 5. 11 column 6). The possible sources of difference are sampling
errors, P content variation within the sludge matrix and soil heterogeneity.
The main reason for the accumulation of P in the soil profile was the low sludge
N:P ratio of the sludge compared with that of crops. Therefore, P accumulation is
inevitable if sludge is applied according to crop N demand (Shober and Sims,
2003).
5.4.2 Soil profile residual Bray-1 extractable P
Bray-1P decreased as the sludge application rate increased to 8 Mg ha-1 yr-1, but
increased at higher rates (Table 5.12). Four years of sludge application according
to the limit reduced residual Bray-1P by 42%. This is in contrast to the net
positive total P mass balance reported for the same treatment on Table 5.11. On
the other hand, sludge applied at double the limit increased residual Bray-1P in
the top 0.5 m by 33 and 58%, compared with the control and 8 Mg ha-1 sludge
treatments, respectively.
Generally the mean background Bray-1P concentration of the soil in the 0-0.3 m
soil stratum (65 mg kg-1) was higher than the optimum concentration required for
most crops (25-30 mg kg-1). It was also higher than the concentration for an
204
optimum soil quality (50 mg kg-1) above which the risk for surface water body
pollution increases as reported (Sims and Pierzynski, 2000).
The decline in Bray-1P reported for the 4 and 8 Mg ha-1 sludge treatments was
mainly due to the FeCl3 added to the sludge at the waste treatment plants, which
reduced the solubility of P (Huang et al., 2007; O’Connor et al., 2004; Huang and
Shenker, 2004; Elliott et al., 2002; Maguire et al., 2000). The build up of Bray-1P
reported from doubling of the sludge upper limit is most probably because the P
supply from the sludge exceeded the buffer capacity of the soil and sludge (Elliott
and O’Connor, 2007) or a decline in the sludge’s P-sorbing capacity over time
(Lu and O’Connor, 2001). It is also possible that the Fe-P minerals may have
released P by dissolution as reported by Huang et al. (2007). Therefore, it is
apparent from this study, that although, sludge treated with FeCl3 reduced P
plant availability; it is likely that P availability could also increase at higher rates
over time.
Despite the high background concentration and additional P added with the
sludge, there were no visible phosphorus toxicity symptoms during the study
period. This is most probably due to the high Zn added with the sludge, because
previous studies conducted by Loneragan et al. (1979), Safaya (1976), Parker
(1997), Webb and Loneragan (1988), and Silber et al. (2002), all show that plant
P toxicity is enhanced under Zn deficient conditions.
205
Table 5.12 Residual Bray-1P in the top 0.5 m soil stratum after the second hay
cut of dryland pasture (weeping lovegrass) during the 2004/05 to 2007/08
growing seasons.
Treatment
Control†
4 Mg ha-1 yr-1
8 Mg ha-1 yr-1
16 Mg ha-1 yr-1
Inorganic fertilizer‡
After 2nd hay cut
in 2005/06
kg ha-1
282
253
150
367
276
Initial
274
274
274
274
274
After 2nd hay cut in
2007/08
254
230
159
378
295
† Zero sludge and inorganic fertilizer applied
‡ 200 kg N and 40 kg P per hectare per annum applied
In summary, doubling of the annual sludge upper limit to satisfy crop N demand
enhanced the build up of total P. Furthermore, this study showed that plant
availability of P (Bray-1P) from dryland pasture soils fertilized with surface
applied sludge, treated with Fe-Cl3, is affected by the rate of sludge application.
Consequently, Bray-1P decreased as the sludge application rate increased to 8
Mg ha-1, but increased at higher rates over time.
Therefore, the following hypotheses were accepted:
•
Sludge application according to weeping lovegrass N demand results in
the accumulation of total and Bray-1P in the soil profile.
5.5 Conclusions
Weeping lovegrass hay yield, crude protein content, and rainfall use efficiency
increased with doubling of the annual upper limit sludge application norm for
206
sludge with 2.56% mean N. Highest yield was harvested from years which
experienced high rainfall compared with similar treatments experiencing low
rainfall for all sludge treatments. Weeping lovegrass annual N uptake increased
significantly as the sludge application rate was doubled. Sludge applied
according to the norm of 8 Mg ha-1 yr-1 was not sufficient to satisfy weeping
lovegrass N demand. Doubling the upper sludge limit norm did not cause the
accumulation of nitrate and ammonium in the profile. Sludge applied at double
the norm, however, increased both total and Bray-1P with time.
Therefore, the following hypotheses were accepted under the prevailing climatic
conditions:
1. Sludge application above the 8 Mg ha-1 yr-1 norm could significantly
increase dryland pasture (weeping lovegrass) hay yield, crude protein
content, and water use efficiency.
2. The ideal sludge application rate to satisfy dryland pasture (Weeping
lovegrass) N demand is dynamic and could exceed the 8 Mg ha-1 yr-1
sludge norm.
3. Sludge applications of up to 16 Mg ha-1 yr-1 that produced the highest hay
yield under the prevailing climatic and edaphic conditions are not prone to
excessive nitrate leaching in the medium term (four years).
4. Sludge application according to weeping lovegrass N demand results in
the accumulation of total P.
207
5. Sludge application according to weeping lovegrass N demand could result
in the accumulation of Bray-1P at higher rates.
The following hypothesis was rejected under the prevailing low rainfall conditions
of the study site:
•
Under high hay yield production conditions, N supply from double the norm
can be fully utilized.
Nevertheless, long-term model simulations should be conducted under varying
rainfall, sludge N concentration, sludge type, and soil types in order to get a site
specific real-time ideal sludge loading rate because N mineralization from sludge
could take several years before reaching steady-state conditions.
208
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213
CHAPTER 6
TURFGRASS
The following hypotheses are tested in this chapter.
High sludge surface loading rates well above recommendations based on crop
removal:
1. Are possible without reducing turf growth and quality.
2. Do not cause an accumulation of N and P below the active root zone.
3. Can minimize soil loss through sod harvesting, and
4. Do not cause unacceptably high nitrate and salt leaching.
214
6.1 Turfgrass growth and quality
6.1.1 Establishment rate
Establishment rate was estimated from mean percent basal cover. Sludge
application rates above the 1997 South African upper limit norm (8 Mg ha-1)
significantly improved turf establishment rates (Table 6.1). The rapid rate of
establishment in the high sludge application treatments can be attributed to the
increase in the availability of essential nutrients, especially N and P and/or the low
bulk density of the growing medium. Sludge application rates above agronomic
limits, therefore, could have the advantage of increasing the number of sod
harvests per season by speeding up the rate of turfgrass growth. This opens up
an opportunity to export even more sludge.
6.1.2 Turfgrass colour
Turfgrass colour rating significantly improved with increase in sludge application
rate (Table 6.1). Turfgrass which received sludge rates higher than the former
South African upper limit norm exceeded the ‘acceptable’ colour ratings (7 - 9).
The highest colour rating was scored by the 67 and 100 Mg ha-1 treatments and
the lowest by the zero sludge treatment. The increase in colour rating observed
with increase in sludge application rate was most likely as a result of the increase
in plant available N, as reported by Bilgili and Acikgoz (2005). Nitrate
concentration data from the WFDs installed at 0.3 m (Fig. 6.1) also clearly
illustrated that there was an increase in the availability of N with increase in sludge
215
Table 6.1 Kikuyu (Pennisetum clandestinum Hochst. ex Chiov.) turfgrass sod quality (establishment rates (% mean
vegetative cover), visual colour ratings, and sod integrity) as affected by five sludge application rates during the
2005 and 2006 growing seasons at East Rand Water Care Works, Johannesburg, South Africa.
Days after sludge application
2005
Sludge application
rate
34
Seasonal mean visual
colour rating†
2006
65
34
65
Mean vegetative cover, %
2005
2006
Sod integrity
(percent harvestable
sod per unit area)
2006
2005
Colour ratings
%
Control
34
58
36
68
4.0
3.8
72
71
8 Mg ha-1 per cut
41
74
43
75
4.8
5.3
72
73
33 Mg ha-1 per cut
56
88
59
87
7.0
7.3
93
96
67 Mg ha-1 per cut
63
95
68
95
8.5
8.5
90
88
cut
71
98
71
98
8.5
9.0
58
62
LSD (5%)
6
5
6
6
0.7
0.7
7
7
CV‡, %
7
4
7
5
6
7
6
6
100 Mg ha-1 per
† Visually rated on a scale from 1 to 9 (1 = straw brown; 9 = dark green).
‡ Coefficient of variation
216
-1
NO3 (mg L )
application rate during most of the growing season.
a
300
250
200
150
100
50
0
Control
8 t/ha
33 t/ha
100 t/ha
0
28
56
84
112 140 168 196 224 252
Days after sludge application
-1
NO3- (mg L )
300
250
200
150
100
50
0
b
0 Mg/ha
8 Mg/ha
33 Mg/ha
100 Mg/ha
0
28
56
84
112 140 168 196 224 252
Days after sludge application
Figure 6.1 Concentration of nitrate in soil solution samples collected from wetting
front detectors installed at 0.30 m of a turfgrass sod (Pennisetum clandestinum)
trial for four sludge application rates (0 Mg ha-1, 8 Mg ha-1, 33 Mg ha-1, and 100
Mg ha-1) during (a) year 2005 and (b) 2006.
217
6.1.3 Sod integrity
The percentage of sods that did not break during handling increased as the
sludge application rate increased to 33 Mg ha-1, but decreased at higher rates
(Table 6.1). The 0 Mg ha-1 sludge treatment produced heavy sods up to 54 Mg
ha-1 heavier than the treatment receiving 100 Mg ha-1. An increase in the sludge
application rate to 33 Mg ha-1 reduced sod mass and the more vigorous turf was
able to bind the soil/sludge mix effectively. However, as the sludge application
rate increased to 67 and 100 Mg ha-1, the sod became weaker and a greater
proportion fell apart during handling and loading. Nevertheless, sod integrity was
still good at 67 Mg ha-1, and was not significantly different from the 33 Mg ha-1
treatment in 2005 (93 vs. 90%). Although significant in 2006 (96 vs. 88%), the
difference was only 8%. Highest percentages of strong intact sods were
harvested from the 33 Mg ha-1 treatment, and the lowest from the 100 Mg ha-1
treatment for both seasons.
One may expect that the strategy of harvesting sods once the slowest growing
treatments had reached full cover may also have contributed to differences in sod
integrity among the treatments as a result of differences in maturity. However, it
is interesting to note that the slowest growing treatments (0 and 8 Mg ha-1) did
not have the weakest sods, as the high fraction of soil in the sod matrix,
increased their integrity relative to the 100 Mg ha-1 treatment, which had a very
high fraction of sludge in the sod.
218
The growth study showed that establishment and colour continued to improve up
to an application of 67 to 100 Mg ha-1, well above the maximum recommended
limit, but that sod integrity decreased after 33 Mg ha-1.
6.2 Accumulation of N and P in soil below active root zone
6.2.1 Nitrogen
The mass of N exported with the sod increased significantly with increase in
sludge application rate (Table 6.2), with 86 to 91% in the substrate fraction of the
sod. The amount of N in the plant component of the sod also increased
significantly with increase in application rate, although plant N contributed only
10-15% of the total sod N. Similarly, Vietor et al. (2002) reported the export of a
large N fraction in the soil component of the sod. Excess N exported with the sod
can provide nutrition for turfgrass at the new site of establishment, where the
original topsoil has often been removed due to construction. It was also evident
that a significant amount of N was removed with clippings.
The total N content within the top meter of soil declined over the two year period
in all treatments receiving less than 100 Mg ha-1 sludge (Table 6.3). Based on
soil sampling over this depth (change in soil storage), sludge applications of
around 67 Mg ha-1 were approximately in balance for nitrogen (Table 6.3).
Generally a mass balance difference was observed in the top 1 m soil layer for all
treatments. This could be attributed to leaching (Sierra et al., 2001; Samaras et
219
al., 2008), volatilization (Beauchamp et al., 1978; Robinson and Polglase, 2000),
and denitrification losses (Monnett et al., 1995). Considering the high leaching
fraction experienced in this study, the largest loss was most likely through
leaching. Sampling errors, N content variation within the sludge matrix, and soil
heterogeneity may also have contributed to the differences.
Total soil N content decreased with depth for all treatments (Fig. 6.2a). After two
years, the total N content of the top 0.15 m layer was less than the initial total N
content of the same layer for all treatments. This could be due to grass uptake,
leaching to lower layers, and the removal of a thin surface layer of soil during sod
harvesting, which is rich in organic matter. Significant accumulation of N was
evident in the 0.15-0.75 m layers of the 100 Mg ha-1 and the 0.45-0.75 m layers
of the 67 Mg ha-1 treatments (Fig. 6.2a), despite the decline in the top 0.15 m
layer. This indicates that N was leached to the lower layers because of the high
leaching fraction experienced during the study period.
Soil nitrate concentration is a highly dynamic property, reflecting the size of the
labile N pool and the antecedent conditions favouring mineralization and
leaching. Sludge application rates lower than 33 Mg ha-1 showed a marked
depletion in nitrate levels by the end of the study (Fig. 6.2b). Rates of 33 Mg ha-1
and above, maintained the nitrate concentration of the soil profile close to initial
values, which as a benchmark, were within the ranges reported for golf courses
from a lysimeter study (Wong et al., 1998).
220
Table 6.2. Total N imported with sludge, vs. exported with sods and clippings during the 2004/05 and 2005/06
growing seasons at East Rand Water Care Works, Johannesburg, South Africa.
N imported with sludge vs. exported with sod and clippings
Exported
Imported
Sludge
application
rate
2005
2006
Sod
2005
2006
Soil
Sod
Plant
Clippings
Mg ha-1
Total
Soil
Plant
Clippings
Total
kg ha-1
0
0
0
400
66
174
640
297
37
154
488
8
243
151
466
77
268
811
358
47
255
660
33
1001
622
773
96
487
1356
655
68
419
1142
67
2031
1262
1050
141
620
1811
798
100
582
1480
100
3032
1884
1104
143
797
2044
1094
143
712
1949
70
10
30
78
56
4
30
61
6
7
6
5
6
3
5
5
LSD (5%)
CV‡, %
† Means in the same column followed by the same letter are not significantly different at P < 0.05 level.
‡ Coefficient of variation
221
Table 6.3 Total nitrogen and total phosphorus mass balances after two years of sludge application and sod harvest
events for five sludge application rates during the 2005 and 2006 growing seasons
Nitrogen
Change in
Sludge
application
rate
Imports
Exports
Mass
soil
balance
storage†
difference‡
Imports
Mass
soil
balance
Exports
storage†
difference‡
kg ha-1
-1
Mg ha
0
Phosphorus
Change in
0
1128
-1295
-167
0
459
-465
-6
394
1471
-1318
-241
305
713
-452
-44
33
1623
2498
-1116
-241
1257
1389
-165
-33
67
3293
3291
-254
-256
2551
2349
175
-27
100
4916
3993
740
-183
3806
3479
291
-36
8
† Estimated by subtracting the initial soil profile N or P content before treatment application from the soil profile N or
P content at the end of the two year study period (2006).
‡ Estimated by subtracting the change in soil storage from the (Imports – Exports).
222
The concentration of ammonium was elevated at applications greater than 33 Mg
ha-1 (Fig. 6.2c). This was probably due to adsorption to CEC sites, as the soil had
a high and fairly uniform clay content (34%-40%) to a depth of 1.2 m.
0
Total soil N (mg kg-1)
200 400 600 800
1000
0
50
0
0.2
a
0.4
0 Mg/ha
8 Mg/ha
33 Mg/ha
67 Mg/ha
100 Mg/ha
Initial
0.6
0.8
1
Sampling depth (m)
0
Sampling depth (m)
10
Soil NO3- (mg kg-1)
20
30
40
b
0.2
0.4
0 Mg/ha
8 Mg/ha
33 Mg/ha
67 Mg/ha
100 Mg/ha
Initial
0.6
0.8
1
1.2
1.2
0
10
Soil NH4
20
+
-1
(mg kg )
30
40
50
0
Sampling depth (m)
0.2
c
0.4
0 Mg/ha
8 Mg/ha
33 Mg/ha
67 Mg/ha
100 Mg/ha
Initial
0.6
0.8
1
1.2
Figure 6.2 Soil profile (a) total N (b) nitrate (c) ammonium (d) total P (e) Bray-1
extractable P, and (f) electrical conductivity (ECe) as affected by two consecutive
years of sludge application at five rates (0, 8, 33, 67, and 100 Mg ha-1) in a
turfgrass sod (Pennisetum clandestinum) field trial, sampled before treatment
application in 2005 (initial) and after two sod harvests in 2006.
223
6.2.2 Phosphorus
The mass of P exported with the sod increased significantly as the sludge
application rate increased (Table 6.4), with 93% to 99% in the substrate fraction
of the sod. Phosphorus stored in the soil profile declined over the two year period
in all treatments receiving less than 67 Mg ha-1 of sludge, based on soil sampling
over the 1.0 m depth (Table 6.3). Based on this sampling depth (change in soil
storage), sludge application rates of 33 to 67 Mg ha-1 were approximately in
balance for phosphorus. However, the 67 Mg ha-1 treatment was associated with
a net positive P accumulation (175 kg ha-1) within two years. A mass balance
difference was evident for all treatments and could most probably be from
sampling errors, P content variation within the sludge matrix, and soil
heterogeneity.
Soil profile total P (mg kg-1) and Bray-1 extractable P (mg kg-1) decreased with
depth for all treatments (Figs. 6.3a and 6.3b). Sludge application rates higher
than 33 Mg ha-1 resulted in the build up of total P in the top 0.15 m soil layer.
However, the concentrations of Bray-1 extractable P for all treatments were less
than the initial soil profile concentration (before treatment application) (Fig. 6.3b).
The build up of total P in the top 0.15 m soil layer of the 67 and 100 Mg ha-1 rates
may be from some sludge remaining after sod lifting or from preferential flow of
particulate P (Jensen et al., 2000; Brock et al., 2007).
224
1
Table 6.4 Total phosphorus imported with sludge, vs. exported with sods and clippings during the 2005 and 2006 growing
seasons at East Rand Water Care Works, Johannesburg, South Africa.
2
P imported with sludge vs. exported with sod and clippings
Exported
Imported
Sludge
application
rate
2005
2005
2006
Sod
Soil
Plant
2006
Clippings
Mg ha-1
Sod
Soil
Plant
Clippings
Total
kg ha-1
0
0
0
229
16
13
258
170
19
12
201
8
157
148
358
18
21
397
275
21
20
316
33
648
609
661
18
48
727
598
22
42
662
67
1315
1236
1148
18
53
1219
1056
25
49
1130
100
1962
1844
1782
21
112
1915
1440
26
98
1564
108
1
3
108
59
1
3
59
6
5
4
4
6
5
6
5
LSD (5%)
CV†, %
3
Total
† Coefficient of variation
225
Total soil P (mg kg-1)
0
200
400
600
800
1000
Sampling depth (m)
0
a
0.2
0.4
0 Mg/ha
0.6
8 Mg/ha
0.8
33 Mg/ha
1
1.2
Bray-1 extractable P (mg kg-1)
0
5
10
15
Sampling depth (m)
0
0.2
b
0.4
0.6
0 Mg/ha
8 Mg/ha
33 Mg/ha
67 Mg/ha
100 Mg/ha
Initial
0.8
1
1.2
Figure 6.3 Soil profile (a) total P (b) Bray-1 extractable P as affected by two
consecutive years of sludge application at five rates (0, 8, 33, 67, and 100 Mg ha1
) in a turfgrass sod (Pennisetum clandestinum) field trial, sampled before
treatment application in 2005 (initial) and after two sod harvests in 2006.
226
The Bray-1 extractable P concentrations of all treatments were below the
optimum concentration of 25 to 30 mg kg-1 required for most crops (Sims and
Pierzynski, 2000) and below the ‘low sufficiency’ range of 6-12 mg kg-1 for the
common soil Bray-1P test reported by Havlin et al. (2005). The decline in Bray-1
extractable P from all sludge loadings of less than 67 Mg ha-1 could most
probably be due to plant uptake. For treatments receiving sludge rates higher
than 67 Mg ha-1, however, it might also be caused by the Fe added with the
sludge. This is because the sludge used in this study has gone through tertiary
treatment, and is rich in insoluble Fe-P compounds. Chemicals used in tertiary
treatments such as Al or Fe salts, decrease the labile P fraction (Elliott et al.,
2002; Häni, et al., 1981; Kyle and McClintock, 1995).
In summary, total N within the top meter of soil decreased over the duration of
the trial for rates not exceeding 67 Mg ha-1. Total P content within the same
depth also declined for rates not exceeding 33 Mg ha-1. Bray-1 extractable P,
however, decreased for all treatments. Based on soil sampling over a depth of
1.0 m, sludge applications of around 67 Mg ha-1 approximately preserved N mass
balance, but was associated with significant N leaching to the lower 0.45-0.75 m
soil layer. The P mass balance was, however, preserved at loading rates of 33
Mg ha-1. Total P accumulation was evident mainly in the top 0.15 m of treatments
which received sludge loading rates higher than 33 Mg ha-1 under the prevailing
management practices and climate.
227
6.3 Soil loss through sod harvesting
Soil loss from the site was estimated from the soil depth exported with the sod.
Results from this study show that severe soil loss takes place in turfgrass sod
production areas where little or no amendment is applied (Table 6.5). The depth of
soil exported from the site decreased significantly with increasing sludge
application rates above 8 Mg ha-1. The application of 33, 67, and 100 Mg ha-1
saved 120, 217, and 290 Mg ha-1 soil respectively from being exported from the
site within two years (assuming a soil bulk density of 1380 kg m-3). Considering the
very slow average global rate of soil formation, 700 kg ha-1 yr-1, (Wakatsuki and
Rasyidin, 1992) the 33, 67, and 100 Mg ha-1 treatments saved soil from being
exported which could have taken 86, 155, and 207 years to form, respectively.
Sod mass decreased for every increment in sludge application rate because of the
low bulk density of sludge (666 kg m-3) compared with soil (Table 6.5). This also
has a direct financial implication for sod transportation. Thus sludge applied above
agronomic limits, will also reduce sod transportation costs.
In summary, the application of sludge did help to reduce the rate of soil loss
through sod harvesting and should reduce transportation cost through lowering
sod mass. Rates as high as 100 Mg ha-1 were required to completely avoid soil
loss. However, such high loading rates deleteriously affected sod integrity.
228
Table 6.5. Sod mass and cumulative soil thickness exported with turfgrass sods
as affected by five sludge application rates after two consecutive sludge
application and sod harvest events at East Rand Water Care Works,
Johannesburg, South Africa.
Sod mass
Sludge application
rate
2005
2006
Mg ha-1
Cumulative
Soil thickness exported
(2005-2006)
mm
0
149
156
22.0
8
147
155
21.5
33
141
148
13.3
67
112
128
6.3
100
99
102
1.0
LSD (5%)
9
10
1.6
CV†, %
† Coefficient of variation
4
5
8
6.4 Nitrate and salt leaching
Soil solution nitrate and EC measurements were made on samples collected by
the 0.3 m deep WFDs during the first three months after sludge application in
2005, and throughout the 2006 study period. Because the 67 Mg ha-1 treatment
did not have WFDs, this treatment is excluded from the discussion that follows.
229
6.4.1 Nitrate leaching
In the beginning of the season, soil solution nitrate concentration increased with
sludge application rate (Fig. 6.1). This is to be expected as nutrient supply far
exceeds demand directly after sod harvest, as explained in detail by Geron et al.
(1993). Later during the season, the concentration of nitrate remained at low
levels (Fig. 6.1), presumably because the greater demand from the turf matched
the mineralization rate from the sludge. Similar results and trends were recorded
both during the 2004/05 (Fig. 6.1a) and 2005/06 (Fig. 6.1b) growing seasons.
Generally, the concentrations of nitrate in the soil solution from all treatments
were within the ranges reported by Biró et al. (2005) for leachate from organic
and conventionally managed horticultural lands (0-255 mg L-1 NO3-). It was also
less than the maximum nitrate concentration in leachate from a simulated golf
green (376 mg L-1 NO3-) reported by Shuman (2001).
The concentration of nitrate leachate from all treatments remained higher than
South African drinking water standards (44 mg NO3- L-1) (Korentajer, 1991) for
the first two to three months in the 100 Mg ha-1 sludge treatment (Fig. 6.1). The
wetting front detectors installed at 0.6 m depth, however, did not collect soil
solution samples from any of the treatments. Therefore, it was not clear whether
the nitrate that passed the WFD at 0.3 m, had leached below the WFD at 0.6 m,
perhaps in a weak front below the detection level of the WFD, or was stored
between the two depths.
230
The seasonal average nitrate leachate concentrations for the 0, 8, and 33 Mg ha-1
sludge treatments (26, 29, and 43 mg NO3- L-1 respectively) were less than the
South African drinking water standard (44 mg L-1) (Korentajer, 1991) and the EU
nitrate concentration limit for groundwater (50 mg NO3- L-1) (Vlassak and Agenbag,
1999). The seasonal average nitrate concentration for the 100 Mg ha-1 treatment
(63 mg NO3- L-1), however, exceeded both limits. Compared with the zero sludge
treatment, the addition of 8, 33, and 100 Mg ha-1 sludge increased the average
seasonal nitrate concentration of the leachate by 3, 17 and 36 mg NO3- L-1
respectively.
6.4.2. Salt leaching
In the beginning of the season soil solution EC increased with sludge application
rate (Fig. 6.4). The EC of soil solution samples from the 100 Mg ha-1 treatment
collected 10 days after sludge application were only slightly higher than the
threshold value with 10% yield reduction of 300 mS m-1 for kikuyu (Yiasoumi et
al., 2005). Nevertheless, highest mean percent vegetative cover was recorded for
this treatment (Table 6.1). Treatments receiving less than 100 Mg ha-1 sludge,
however, had lower soil solution EC values than this threshold (Figs. 6.4a and b).
For soil solution samples from the 33 and 100 Mg ha-1 treatments, the EC
dropped very fast at the beginning of the season. This indicates that most of the
salts added through the sludge were leached below the active root zone during
the first 60 to 84 days after application. This was mainly because of the high
231
leaching fraction (0.27 in 2004/05 and 0.3 in 2005/06) experienced during those
periods from irrigation and rainfall.
An increase in soil salinity following sludge application is inevitable and was also
observed in the studies conducted by Navas et al. (1998) and Stamatiadis et al.
(1999). Nonetheless, the soil salinity levels of all the treatments at the end of the
trial (Fig. 6.5) were much lower than the threshold value of 300 mS m-1 for kikuyu
production (Yiasoumi et al., 2005). At the end of the trial, application rates higher
than 33 Mg ha-1 significantly increased soil salinity, but the levels were still
acceptably low (Fig 6.5).
In summary, the requirement to leach salts is the most difficult aspect of
managing large volume sludge applications. Nitrate leaching cannot be quantified
in this study, but based on the solutions collected at 0.3 m depth and the change
in soil N over the two seasons (Table 6.2), it would have been significant. Based
on the EC of soil water and the growth studies, the leaching fraction could have
been reduced. It may also be necessary to apply the sludge in two applications,
and delay the second application until low nitrate was measured in the wetting
front detectors. This high leaching fraction, however, could simulate worst case
scenarios.
232
-1
EC (mS m )
350
300
250
200
150
100
50
0
a
Control
8 t/ha
33 t/ha
100 t/ha
0
28
56
84 112 140 168 196 224 252
-1
EC (mS m )
Days after sludge application
b
350
300
250
200
150
100
50
0
0 Mg/ha
8 Mg/ha
33 Mg/ha
100 Mg/ha
0
28
56
84 112 140 168 196 224 252
Days after sludge application
Figure 6.4 Electrical conductivity of soil solution samples collected from wetting
front detectors installed at 0.30 m of a turfgrass sod (Pennisetum clandestinum)
trial for four sludge application rates (0 Mg ha-1, 8 Mg ha-1, 33 Mg ha-1, and 100
Mg ha-1) during (a) year 2004/05 and (b) 2005/06 growing seasons.
233
EC (mS m-1)
0
20
40
60
80
Sampling depth (m)
0
0.2
0.4
0 Mg/ha
8 Mg/ha
33 Mg/ha
67 Mg/ha
100 Mg/ha
Initial
0.6
0.8
1
1.2
Figure 6.5 Soil profile electrical conductivity as affected by two consecutive years
of sludge application at five rates (0, 8, 33, 67, and 100 Mg ha-1) in a turfgrass
sod (Pennisetum clandestinum) field trial, sampled before treatment application
in 2004/05 (initial) and after two sod harvests in 2005/06.
6.5 Conclusions
The study demonstrated several advantages to large volume sludge loading of
turf for sod production. Establishment rate and turf colour improved up to 67 Mg
ha-1. Very high loadings of 100 Mg ha-1 had no deleterious effect on the turf
growth, despite the salt content of the sludge. High loadings also produced lighter
234
sods, which could reduce transport costs, and minimize the loss of soil from the
site. However, very high rates also drastically reduced sod integrity.
Soil sampling before and after the two year trial showed that N and P did not
accumulate below a depth of 0.45 m until more than 33 Mg ha-1 sludge was
applied. At the end of the study the available N and P were relatively low, and
would not constitute a threat to groundwater in this specific soil type.
The disadvantages of excess sludge loading are apparent when comparing the N
and P mass balance with the actual change as measured by soil sampling.
Clearly there are large losses of N and P which are hard to manage.
In
particular, the requirement to leach salt makes it difficult to control the leaching of
nitrate, although in this study a lower leaching fraction would have been possible.
Application rates higher than 67 Mg ha-1 would be unacceptable on
environmental grounds, but they were also unacceptable for reasons of turf
quality, as the sods tended to break apart during handling.
An application rate of 33 Mg ha-1, four times higher than the 1997 South African
upper recommended limit or more than three times the new guideline, would be
an acceptable compromise. This rate improved growth and sod quality over the
zero and 8 Mg ha-1 application treatments. Great care would be required to
minimize nitrate leaching during the first two months. Leaching could be
managed by allowing the EC of leachate captured by the wetting front detectors
235
not to exceed the threshold value of 300 mS m-1. In other words, the crop
coefficient could be reduced, and only increased when high salt levels were
recorded. Applications greater than 33 Mg ha-1 could be possible if the sludge
applications were split by delaying the second application until low nitrate was
measured in the wetting front detectors, as long as turf quality was not negatively
affected.
Therefore, all hypotheses were accepted for application rates not exceeding 33
Mg ha-1, on the proviso that some soil loss was acceptable and that the leaching
fraction was carefully managed during the first two months after sludge
application. Further research is needed for other soil types and possible runoff
losses from fields with some gradient.
236
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